Oil sand generally comprises water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules that contain a significant amount of sulfur, nitrogen and oxygen. The extraction of bitumen from sand using hot water processes yields large volumes of fine tailings composed of fine silts, clays, residual bitumen and water. Mineral fractions with a particle diameter less than 44 microns are referred to as “fines”. These fines are typically clay mineral suspensions, predominantly kaolinite and illite.
The fine tailings suspension is typically 85% water and 15% fine particles by mass. Dewatering of fine tailings occurs very slowly.
Generally, the fine tailings are discharged into a storage pond for settling and dewatering. When first discharged in the pond, the very low solids content material is referred to as thin fine tailings. After a few years, the tailings separate into an upper layer of water, a settled layer of coarse solids and a fluid fine tailings (FFT) layer between the upper water layer and the bottom layer of settled coarse solids. The fluid fine tailings generally have a solids content of about 10-45 wt % and behave as a fluid-like colloidal material.
A substantial amount of bitumen remains in the tailings stream from oil sand extraction. For example, there is approximately 20 MBbl of bitumen per 100 Mm3 of fluid fine tailings.
The tailings bitumen represents a large loss given the commercial value of this potentially useable hydrocarbon. Furthermore, the tailings bitumen interferes with tailings operations, including reducing the efficiency of tailings treatments. In addition, tailings bitumen may represent an environmental risk by accumulation in the storage ponds.
Accordingly, there is a need for a method to recover bitumen from tailings in the storage ponds.